Cook the pasta in abundantly salted water until al dente.

While you are cooking your spaghetti, whisk the egg with grated pecorino cheese and lots of freshly ground pepper. (For most palates, if you salt the pasta water sufficiently and given that pecorino is already quite salty, you probably don't need to season the sauce.)

Separately, in a skillet, sauté the garlic until it is just slightly brown. (If using onion, let it sauté very gently until it is perfectly soft and translucent, without browning.) Then add the zucchini and continue over moderate heat, seasoning with a pinch of salt, until the zucchini is cooked and just lightly browned. If you're using young, tender zucchini, this should take 5 minutes or less.

Once the pasta is cooked al dente, drain them (but not too well) and pour them into the skillet over very low heat. Make sure there is a bit of pasta water clinging to the pasta; if not, add a ladleful from the pasta pot. Mix well and then add your egg mixture and mix again. Keep mixing until the eggs just being to thicken and form a creamy sauce that clings to the pasta. If you prefer (and these days I tend to like my carbonara this way) or if you are worried about contamination, you can continue a bit longer until the eggs actually set. In either case, remove from the heat as soon as it just bit less done than you want, as the eggs will continue to cook from the residual heat of the pasta.

Serve immediately in warm pasta plates, with additional pecorino and ground pepper for those who want some.